1. Field of the Invention
The present invention relates to drilling oil and gas wells. In particular, the present invention relates to liner hanger systems for use in providing cemented zonal isolation via lining casing strings in oil and gas wells.
2. Description of the Related Art
Oil wells typically have casing strings, or liners, installed prior to penetrating a reservoir and beginning production of oil and gas. Such casing strings provide weight hanging capability and pressure zonal isolation. In some instances, it may be necessary to run more than one casing string. In such instances, the first casing string, installed at the top of the well, has the largest diameter. Thereafter, a subsequent casing string is placed into the well by passing it through the already installed first casing string.
As the subsequent section of casing string is positioned in the well, and in particular at the top of a reservoir, it is typically sealed and locked to the already installed casing string via a liner hanger assembly. In addition, cement is typically pumped down the well, through the drillpipe and liner string, and then back up the well on the outside of the liner string. Proper hanging and cementing of liner string is important to isolate the production path within the liner string. Failure to install the liner string properly can lead to costly lost rig time, additional well repair cost, and wellbore pressure integrity problems later on. Although many conventional liner hanger assemblies have been developed, there are still repeated failures reported in the industry, such as, for example, leaks across the liner top packer, and problems with naming and setting tools.
One element for the installation of liner is the expandable liner hanger. In an expandable liner hanger, an inner casing string is run into position in the wellbore through a previously installed outer casing string. Once in place, an upper end of the inner casing string is expanded into contact with the inside walls of the outer casing string at a depth typically a couple of hundred feet above its shoe. The inner casing string diameter can be expanded by forcing an expansion cone into the top of the outer casing string.
Because the expansion cone is commonly located at the bottom of the polished bore receptacle (PBR) or tie-back receptacle (TBR), which is located immediately above the expandable casing string and sealing elements, the area for cement slurry flow is restricted. In wells having high formation pressure, which require high mud weight and high density cement slurry, there is an increased risk of incurring losses of cement slurry during liner cementing operations. This is, to a large extent, because of the high equivalent circulating density due to rather narrow clearances between the outer-diameter of the expansion cone and setting tools housing, and the outer casing. Consequently, voids or poor cement bond between the outer casing and liner and/or between the liner and rock formation may develop, which could lead to possible leak paths, or poor zonal isolation.